Astigmatism, an optical defect which blurs vision, is usually caused by an irregular curvature of the cornea. In a perfectly shaped eye, the cornea is spherical, and bends all incoming light in the same way, producing a sharply focused image on the back of your eye (retina). In individuals with astigmatism, the cornea is asymmetrically curved, similar to the surface of a football, with one axis of the ball curved more steeply than the opposite axis of the ball. This causes the light rays to be bent differently, which causes the image to be blurred.
The blurred vision from the astigmatism can be measured and is designated as the astigmatic refractive error. The astigmatic refractive error is measured in terms of its power and axis. The astigmatic power is measured in diopters, and the axis is measured in degrees representing the direction on a 360 degree scale. The axis reflects the direction of the steepest or flattest meridian of the cornea. The axis of astigmatism is unique to each individual.
The goal of treating astigmatism is to address the uneven curvature that causes blurred vision. Astigmatism may be corrected with eyeglasses, contact lenses, or surgery. Surgical correction of astigmatism can include laser (such as Lasik or Prk) as well as LRI (Limbal Relaxing Incisions, which are deep incisions in the peripheral cornea that flatten the steep meridian) and astigmatic intraocular lenses (at the time of cataract surgery).
A cataract is a condition where the normally clear lens of the eye becomes progressively opaque. This opacification typically occurs over an extended period of time, and the amount of light that passes through the lens decreases with increasing degrees of opacity. As the ability of the cataract lens to transmit light decreases the ability of the eye to perceive images also decreases. Blindness ultimately can result. Since there are no known methods for eliminating the opacity of a cataract lens, it generally is necessary to surgically remove the opaque lens to permit the unobstructed passage of light through the pupil to the retina.
In cataract surgery, the cloudy natural lens is removed from the eye. The focusing power of the natural lens can be restored by replacing it with a permanent artificial lens or intraocular lens (IOL) implant. These lenses are placed in the eye and thus closely simulate the optics of the natural lens which they are replacing. During cataract surgery, the astigmatic error can be corrected by either performing Limbal Relaxing Incisions in the peripheral cornea to flatten the steeper meridian, or by inserting a specialized IOL which can correct the astigmatism (a toric IOL).
Regardless of whether astigmatism is corrected with an LRI or with a toric intraocular lens, it is essential to align the surgical intervention with the exact astigmatic axis. Prior to the surgery it is necessary to mark the astigmatic axis onto a patient's cornea as accurately as possible.
A variety of methods and instruments are currently used to mark the cornea prior to surgery. Most involve a small handheld instrument (e.g., U.S. Pat. No. 6,217,596) that is pressed against the cornea prior to surgery, marking reference horizontal or vertical axis. After the eye surgery has started, and the patient is lying down, the reference marks are used to mark the cornea a second time at the direction of the astigmatic axis. This two-step methodology has several shortcomings and introduces several significant sources of error. In particular, error is introduced when the surgeon uses the alignment reference marks made in the first step to then mark the astigmatic axis in the second step. While the alignment is done while the patient is sitting upright, the marking is done while the patient is lying down. The eye undergoes movement within the socket comprising translation and rotation (“cyclotorsion”) as the patient is moved from the upright measuring position to the prone surgery position. Multiple techniques known in the art to accommodate this movement include those disclosed in U.S. Pat. No. 4,476,862 and U.S. Pat. No. 4,705,035. If the eye movement is not taken into consideration when the patient lies down the direction of the axis of astigmatism will not be properly accounted for. Mathematically, missing the axis of astigmatism to be treated by 10 to 15 degrees can lead to a treatment under-correction of 50% or more.
Finally, the change in orientation encountered with the patient lying down with surgeon now approaching the patient from the side or from the forehead can be inherently confusing when attempting to mark the axis which was measured with the patient sitting. Difficulty with centration and rotation compound the multiple errors already introduced.
One system and method that avoids many of these problems is described in U.S. patent application Ser. No. 12/047,261, which is directed to a corneal marking device assembly mounted onto a trial frame apparatus that can be adjusted by using the adjustment features on the trial frame apparatus. The corneal marking device assembly provides the stabilized structure through which the surgeon can observe the eye, measure characteristic features of the eye and mark the corneal surface with a corneal marking device housed in the corneal marking device assembly. While this system reduces disadvantages associated with traditional hand-held devices, however, the use of a trial frame apparatus can be cumbersome.
Accordingly, there is a need for a non-cumbersome system and method that reduces errors associated with the traditional two-step methodology for marking the astigmatic axis on the cornea.